Re: [Algorithms] How to get 3dvector largest coordinate index?

[Marc H. <ma...@gm...>]

  Oops, I did forget the fabs, I wrote the sketch in email, then wrote
a version for testing on my PC.

  As for the 3, I explained that in the message.  There's 8 possible
values for 3 bits, but only 6 are possible configurations of the
comparisons.

On Sun, Mar 1, 2009 at 1:42 PM, Matt J <mjo...@gm...> wrote:
>
> Your lookup table varies from 0..3, e.g. 0, 1, 2, 3, but there are only 3
> components total, not 4, so I'm not sure how you calculated it
>
> Also, you need to look at the absolute value, which you need to determinate
> the largest component in magnitude
>
>>
>>  This problem felt a lot like you could somehow just do math on the
>> conditionals and get the answer.  The second best I could come up with
>> is:
>>
>>  (Course, conditional moves makes much of this moot, but on with the show)
>>
>> //I used 3 for values that can never happen for fun, always cool to go
>> 1 past the end of an array
>> //Replace with 0, 1, or 2 in shipping code, assert in debug
>> static uint s_lookup[] = {
>> /*000*/ 2,
>> /*001*/ 2,
>> /*010*/ 3,
>> /*011*/ 0,
>> /*100*/ 1,
>> /*101*/ 3,
>> /*110*/ 1,
>> /*111*/ 0
>> }
>>
>> inline uint LargestCoordinate(const Vector3d &v)
>> {
>>  uint first_bit = v.x > v.y;
>>  uint second_bit = (v.x > v.z) << 1;
>>  uint third_bit = (v.y > v.z) << 2;
>>
>>  uint index = first_bit | second_bit | third_bit;
>>
>>  return lookup[index];
>> }
>>
>>  Then, I thought, I could just load a single uint, shift right by the
>> index, then and off the low 2 bits:
>> //Replace binary syntax with something to get around C's lack of
>> syntax. (like http://c-faq.com/misc/sd28.html )
>> inline uint LargestCoordinate(const Vector3d &v)
>> {
>>  uint val = 0b0001110100111010;
>>
>>  uint rotate = (v.y > v.z) << 3 | (v.x > v.z) << 2 | (v.x > v.y) << 1;
>>
>>  return ( val >> rotate ) & 0b11;
>> }
>>
>>  I found I needed to use the int trick also, otherwise the (float >
>> float) statement generates a jump, blah.
>>
>>
>>  (Hire me!)
>>
>>
>> --
>> m   h
>>  arc ernandez
>> / these are the days of lethal perfection /
>
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-- 
m   h
  arc ernandez
/ these are the days of lethal perfection /